array_keys() / array_values() / array_column() Since: PHP 4（2000）

// Returns all keys of an array.
array_keys(array, search_value, strict);

// Returns all values of an array.
array_values(array);

// Extracts the values of a specific column from a multidimensional array.
array_column(array, column_key, index_key);

// Swaps the keys and values of an array.
array_flip(array);

<?php
$product = ['name' => 'Laptop', 'price' => 98000, 'stock' => 15];

// Get all keys.
$keys = array_keys($product);
print_r($keys); // Returns a list of keys: 'name', 'price', 'stock'.

// Get all values.
$values = array_values($product);
print_r($values); // Returns a list of values: 'Laptop', 98000, 15.

// Search for keys associated with a specific value.
$scores = ['English' => 80, 'Math' => 95, 'Science' => 80, 'History' => 90];
$subjects = array_keys($scores, 80);
print_r($subjects); // Returns subjects with a score of 80: 'English', 'Science'.

// Extract a specific column from a multidimensional array.
$users = [
    ['id' => 1, 'name' => 'Alice', 'age' => 25],
    ['id' => 2, 'name' => 'Bob', 'age' => 30],
    ['id' => 3, 'name' => 'Carol', 'age' => 20],
];
$names = array_column($users, 'name');
print_r($names); // Returns an array: 'Alice', 'Bob', 'Carol'.

// Specify an index key using the third argument.
$by_id = array_column($users, 'name', 'id');
print_r($by_id); // Returns an associative array keyed by ID.

// Swap keys and values.
$colors = ['r' => 'Red', 'g' => 'Green', 'b' => 'Blue'];
$flipped = array_flip($colors);
print_r($flipped); // Values become keys, and keys become values.

array_keys() not only retrieves a list of keys from an associative array, but also lets you search for keys that have a specific value by passing that value as the second argument. Passing true as the third argument enables strict comparison, including type checking.

array_values() is useful when you want to convert an associative array into a standard numerically indexed array. It is also commonly used to re-index arrays with non-consecutive keys — for example, after calling array_unique() or array_filter().

array_column() is extremely useful for extracting a specific column from multidimensional arrays, such as database result sets or API responses. By specifying a third argument, you can create an associative array keyed by the values of that column.

For merging or splitting arrays, see array_merge() / array_combine(). For searching within arrays, see in_array() / array_search().